Andrew van der Stock has written a strong criticism of PHP's insecurity. Andrew is a seasoned security expert and a major contributor to OWASP, and he states:

"After writing PHP forum software for three years now, I've come to the conclusion that it is basically impossible for normal programmers to write secure PHP code. It takes far too much effort."

He continues, citing specific areas where he thinks PHP is weak and asserting that "PHP must now mature and take on a proper security architecture."

Chris also mentions that some of the reasons Andrew mentions include register_globals, magic_quotes_gpc, and safe_mode - all due to be removed in the latest PHP version (6). Also, be sure to check out the comments on the post for a good bit more information and discussion...